Argh I did exactly the same - moved in with parents to do Masters, and all my friends bar one (who is out working all day) have left for the summer so I'm pretty much on my own and now thoroughly bored with slow dissertation work! I find focusing on starting PhD is a help, and promising myself the evenings off if I do a good day's work is a good motivator. Aside from that, I'd advise watching funny stuff on YouTube and not buying too many chocolate HobNobs...
Not long to go now, anyway, and it's nice to know there are other people out there going through the same thing. Best of luck!

Don't worry about it too much - I moved to a different uni to do postgrad this year and met most people my age through my course. In fact I was really surprised to find that most people on my course were older than me! My uni does tend to put postgrads together in halls and I think a lot of places do as most postgrads want to be with people their own age. But even if yours doesn't most places have some sort of forum or club for postgrads to socialise through. And you're bound to make some good undergrad friends too - most of them won't be much younger than you! Hope that helps a bit :)